This invention pertains generally to the field of micromechanical structures and processes for producing such structures.
Photolithographic techniques similar to those used in semiconductor processing have been applied to the formation of micromechanical and microelectrical mechanical (MEMS) structures. In the lithographic processes used for producing such structures, a particular challenge is the control of the residual strain in the formed structures. For example, residual compressive strain in freed structures such as bridges, cantilevers and diaphragms can result in the unwanted deflection or distortion of such structures. The residual strain is affected by a number of manufacturing parameters, and small changes in such parameters can significantly impact the performance of the micromechanical devices. For example, a capacitive pressure sensor with a diaphragm that is 2 xcexcm thick and 2mm on the side will lose sensitivity by a factor of 25 for residual tensile stress of just 30 MPa. Residual stress is affected not only by manufacturing parameters such as deposition temperature, pressure, precursor concentrations, post-anneal conditions, etc., but also by packaging variables such as die attach materials, and deployment conditions such as operating temperature and humidity. A number of micromachined strain sensors have been developed in the past to monitor this important material property with the dimensional resolution of a few hundreds microns. Although some of these require mechanical actuation (See O. Tabata, K. Kawahata, S. Sugiyama, I. Igarashi, xe2x80x9cMechanical property measurements of thin films using load-deflection of composite rectangular membrane,xe2x80x9d MEMS ""89, pp. 152-156, 1989), most involve passive structures that are designed to deform measurably under the residual stress when they are released from the substrate. The measurement and control of residual strain is particularly significant in the formation of polysilicon microstructures, as carried out, for example, in U.S. Pat. No. 4,897,360, which also discusses strain sensitive structures that may be used to assess the level of residual strain in the formed materials. See also Y. B. Gianchandani and K. Najafi, xe2x80x9cBent-Beam Strain Sensors,xe2x80x9d JMEMS 5(1), pp. 52-58, March, 1996; H. Guckel, D. Burns, C. Rutigliano, E. Lovell, B. Choi, xe2x80x9cDiagnostic microstructures for the measurement of intrinsic strain in thin films,xe2x80x9d J. Micromech. Microeng., 2, pp. 86-95, 1992; M. Mehregany, R. Howe, S. Senturia, xe2x80x9cNovel microstructures for the in situ measurement of the mechanical properties of thin films,xe2x80x9d J. Appl. Phys. 62 (9), pp. 3579-3584, Nov. 1, 1987; L. Lin, R. Howe, A. Pisano, xe2x80x9cA passive in situ micro strain gauge,xe2x80x9d MEMS ""93, pp. 201-206, 1993; L. B. Wilner, xe2x80x9cStrain and strain relief in highly doped silicon,xe2x80x9d Hilton Head ""92, pp. 76-77, June 1992; J. F. L. Goosen, B. P. van Drieenhuisen, P. J. French, R. F. Wolffenbuttel, xe2x80x9cStress measurement structures for micromachined sensors,xe2x80x9d Transducers ""93, July 1993. These deformations are measured visually, sometimes using a micromachined vernier. Although convenient in a laboratory setting, this method is not amenable to high volume manufacturing. More importantly, it renders the device useless for post-packaging or post-deployment readout, eliminating many conceivable applications.
If a strain sensor with electronic readout could be co-fabricated or co-packaged with another device such as an accelerometer, gyroscope, or pressure sensor, the system accuracy can be improved by offering real-time or test-mode calibration over the lifetime of the system. A method for electronic readout has been developed in which a micromachined bridge is deflected by applying a voltage bias to an electrode located under it, generally to the point that the suspension collapses. K. Najafi, K. Suzuki, xe2x80x9cA novel technique and structure for the measurement of intrinsic stress and Young""s modulus of thin films,xe2x80x9d MEMS ""89, pp. 96-97; P. M. Osterberg and S. D. Senturia, xe2x80x9cM-TEST: A test chip for MEMS material property measurement using electrostatically actuated test structures,xe2x80x9d JMEMS 6(2), June 1997, pp. 107-118. Its usage is constrained in some cases: (a) the fabrication process must permit the inclusion of the electrode; (b) the vertical deflection (perpendicular to the substrate) might not provide accurate data for devices such as the accelerometers and gyroscopes that are designed to deflect laterally (in-plane), particularly when the structural material is anisotropic, such as single crystal silicon or polysilicon with a preferential grain orientation; (c) stiction forces may prevent recovery from collapse, raising concerns about the lifetime of the device and the repeatability of a measurement; and (d) these structures generally are not suitable for compressive materials since they may buckle and collapse.
In accordance with the invention, a micromachined strain sensor is provided which can be incorporated with other micromechanical and microelectronic devices on a substrate such as a semiconductor chip. The strain sensor can be incorporated in a sealed package with other microelectrical and micromechanical components with the residual strain monitored electronically from outside the package. The residual strain in the micromechanical structural elements can thus be monitored conveniently and economically in a production environment, and, if desired, can be monitored over the life of the component to account for changes in structural properties of the micromechanical materials due to changes in environmental conditions, such as temperature, as well as effects due to aging. The strain sensors in accordance with the invention are fully compatible with conventional planar micromachining of common micromechanical materials such as polysilicon, without requiring significant additional device forming steps beyond those required for formation of the micromechanical devices.
The micromachined strain sensor of the invention is formed on a substrate, such as a semiconductor wafer, having a top surface. A microstructural beam member of the strain sensor is anchored to the substrate at one position and has a portion which, during formation of the sensor, is freed from the substrate and which extends over the top surface of the substrate. At least one electrically conductive displaceable tine is connected to the microstructural beam member to be displaced by the member as it is freed from the substrate. A mating electrically conductive tine is mounted to the substrate at a position adjacent to the displaceable tine such that a capacitor is formed between the adjacent tines. Preferably, there are a plurality of displaceable tines and a plurality of mating tines, with the sets of displaceable tines and mating tines connected together in parallel to increase the effective overall capacitance. The microstructural member is formed from a microstructural material, such as polysilicon or electroplated metal, which, for example, is deposited on a sacrificial layer on the substrate. Preferably, the intended micromechanical structures are also formed from the same material at the same time. The material from which the microstructural member is formed may have an intrinsic built-in strain, either compressive or tensile, in its as-deposited form on the sacrificial layer. The mating tines are also preferably formed on the sacrificial layer in a position parallel to and adjacent to the displaceable tines at known spaced positions from the displaceable tines. In the typical production of micromechanical devices, the sacrificial layer is etched away, freeing the microstructural member from the substrate except at its anchor position. When so freed, the microstructural member will tend to expand or contract, depending on whether the built-in strain is compressive or tensile, thereby moving the displaceable tines either toward or away from the mating tines and thereby changing the effective capacitance between the sets of displaceable tines and mating tines. The change in capacitance is thus related to the displacement of the displaceable tines and thereby to the built-in strain within the microstructural member.
In one preferred strain sensor configuration, the strain sensor includes an elongated support beam having a longitudinal direction of the beam, with two pairs of microstructural members connected to the support beam, preferably at its ends, at an acute angle thereto, with the ends of the microstructural members opposite to that at which they are connected to the support beam being anchored to the substrate. Preferably, the pairs of microstructural members are connected to the support beam at both the top and bottom ends of the support beam in a xe2x80x9cbentxe2x80x9d V-configuration to fully suspend the support beam above the surface of the substrate when the beam is freed from the substrate. A plurality of displaceable tines preferably extends outwardly from the support beam at a right angle thereto on both sides of the support beam, and mating tines are preferably formed adjacent to the displaceable tines on both sides of the beam. The support beam, displaceable tines, mating tines, and microstructural beam members are formed from a structural material on a sacrificial layer that has been deposited on the top surface of the substrate or are formed in other conventional microprocessing techniques in which the structures are released after being formed. The structural material (e.g., polysilicon) may, for example, be a layer that is patterned and etched to define the various sensor structures using photolithographic techniques, or the structural material may be metal that is electroplated into a patterned photoresist, and other structural materials, such as single crystal silicon and other semiconductors may be appropriately used in accordance with the invention to produce strain sensor structures. When the sacrificial layer is dissolved or etched away, or the various microstructural parts of the sensor are otherwise freed from the surface of the substrate except at the positions at which the microstructural beam members are anchored to the substrate, the beam members will expand or contract depending on whether the strain in the microstructural parts is compressive or tensile, moving the displaceable tines either closer to or further away from the mating tines. In a preferred embodiment of the invention, the mating tines on opposite sides of support beam are preferably located to be most closely adjacent to opposite sides of the displaceable tines so that displacement of the support beam in one direction brings the displaceable tines closer to the mating tines on one side of the support beam and further away from the mating tines on the other side, allowing the resultant change in capacitance between the displaceable tines and the mating tines on each side of the support beam to be determined by a differential capacitance method to minimize common mode parasitics. To further increase the relative displacement of the tines, and thereby the relative change in capacitance for increased sensitivity, it is preferable that the displaceable tines and the mating tines be formed on complementary structures, i.e., the mating tines are themselves displaceable tines formed on a support beam supported by microstructural member beams which are anchored to the substrate, with the microstructural beam members that support adjacent support beams being oriented in opposite directions so that the adjacent support beams are displaced in opposite directions as the various microstructures are released from the substrate.
In a further preferred sensor structure, microstructural member beams are anchored to a substrate and extend to a connection to a support beam at positions such that expansion or contraction of the microstructural beam members when released from the substrate will pivot the support beam and displace tines connected thereto either toward or away from mating tines mounted to the substrate.
Microelectrical-mechanical devices typically are packaged in an enclosure for use. Examples of such devices are accelerometers, gyroscopes, and pressure sensors. The strain sensors of the invention formed with other micromechanical and electrical devices, on the same or a separate substrate, are also encapsulated within the enclosure. Electrical lead wires extend from electrical connection to the displaceable tines (forming one set of plates of the capacitor) and from the mating tines (which, as noted, may themselves also be displaceable), to lead pins which extend from the package. In this manner, the capacitance between the displaceable tines and the mating tines may be measured electrically from outside the package, allowing the strain in the strain sensors, and thus the built-in strain that will be experienced by other micromechanical structures formed on the substrate, to be monitored from outside the package. Such monitoring allows quality control testing of the packages at the point of manufacture and, if desired, continued monitoring of the strain in the micromechanical devices over the lifetime of the devices to allow compensation for variations in strain caused by changes in temperature, other environmental conditions, and normal aging of the devices. By applying a voltage between the displaceable tines and the mating tines, relative movement between these structures and the microstructural beam members supporting the tines can be driven electrostatically, with the resulting change in capacitance as a function of applied voltage allowing a determination of the Young""s modulus for the microstructural members.